EP1332732B1 - Adjustable long bone prosthesis - Google Patents

Abstract

A long bone prosthesis (10) includes upper and lower articulating components (12,14), with the free ends of the components configured to engage a corresponding prosthetic joint. The overall length of the articulating components (12,14) is adjustable to approximate the length of a patient's long bone, such as the humerus. The articulating components (12,14) include a stem (32) and collet (35) arrangement for fixing the relative axial and rotational position of the components. A tapered lock nut (16) is threaded onto the outer threaded surface (41) of the collet portion (35) to provide a pressure engagement of the articulating elements. Various surface features can be implemented among the components of the prosthesis (10) to enhance the pressure fixation, restrict rotation or provide indexed movement of the articulating components (12,14) relative to each other. <IMAGE>

Description

• The present invention relates to an orthopaedic prosthesis. More specifically, the invention concerns a prosthesis for restoring the functionality of an extremity, such as an arm of a patient. The invention is particularly suited for the replacement of a long bone of the patient.
• The later half of the 20^th century has seen a proliferation in the number of human skeletal components that can be replaced by a man-made prosthesis. Over the years these prosthesis have evolved from simply a physical substitution for a bone or a joint, to the more sophisticated fully functional prosthesis. For example, prostheses are well known for the replacement of the shoulder joint or the elbow joint. A modular shoulder prosthesis designed according to US-5314479 can be integrated into the existing glenoid cavity of a patient's shoulder. The prosthesis includes a lower stem that is configured to be embedded within the existing humerus bone of the patient. US-6290725 discloses a modular elbow prosthesis that includes stems for implantation into the intramedullary canal of the humerus and ulna bones. Similar prosthetic joints exist for replacement of the hip, knee, and ankle joints.
• While many devices exists for the replacement of a damaged or defective joint, the substitution of a bone, and particularly a long bone, is much more problematic. While prosthetic phalanges have enjoyed increasing success, the long bones have not been so easily replaced by a prosthesis, particularly to maintain the functionality of the patient's limb.
• The problems with long bone replacement are many and varied. Perhaps the greatest difficulty is the extreme load-bearing nature of the long bones. One approach to this problem has been to supplement, rather than replace, the bone its self. For instance, as shown in US-4384373, an artificial femoral diaphysis is extended through a longitudinal bore in the femur. The artificial diaphysis is bounded at one end by an artificial knee component and at its opposite end by an artificial hip component. This prosthesis of US-4384373 retains the existing bone and relies upon that bone for a certain amount of load-bearing capability.
• A further problem associated with a long bone prosthesis is the variability in length of a particular long bone between patients. The femur, tibia, radius, and humerus bones vary in length as much as patients vary in height. The aforementioned '373 patent dealt with this variation by providing a motion screw and an external threaded sleeve feature for adjusting the overall distance between the hip joint and knee joint components of the prosthesis. Many similar approaches have been implemented for prosthetic joints. It should be noted that with these prostheses, the joint is affixed to an existing long bone, such as by implantation of a stem into the intramedullary canal of the bone. Documents which disclose a variety of adjustable length features include US-4502160, US-4892546, US-5358524, US-5387239 and US-5906644. These adjustable length features range from the bevel gear and external key arrangement disclosed in US-4502160, to the Morse taper pressure lock feature of the device disclosed in US-5906644.
• One problem common to all of these prior adjustable length features is that the length of the prosthesis cannot be adjustedin situ. In other words, in each of the prior devices, the required length of the prosthesis is predetermined and the prosthesis adjusted before implantation into the patient. In a typical surgical procedure, the affected limb is x-rayed to determine the desired length for the prosthesis. However, in some cases no matter how accurate the x-ray and resulting measurements, certain errors creep in that make accurate measurement of the length of the long bone difficult. For instance, it is often particularly difficult to obtain an accurate x-ray of the upper arm, or humerus bone, of a patient. In a typical scenario, the bone cannot be oriented perfectly parallel with the x-ray plate due to the presence of muscle and fatty tissue around the bone, and other biomechanical disruptions. Thus, often the humerus bone is frequently oriented in a non-parallel relation to the x-ray photographic plate, and most frequently angled slightly outward form the x-ray plate. With this parallax error, it is virtually impossible to accurately determine the length of the humerus bone.
• Inaccurate measurement of the long bone prosthesis of course leads to an improper length of the extremity. This mal-adjusted length can cause motor and muscular difficulties. Moreover, an incorrect prosthesis length can affect the tightness of the tissue surrounding the prosthesis. If the length is too short, the surrounding tissue is unnecessarily loose. If the prosthesis is too long, the tissue may be too tight, if the prosthesis can be implanted at all.
• DE-A-3605630 discloses an endoprosthesis having proximal and distal connecting parts, with a rod shaped part between them. The ends of the rod shaped part are provided with screw threads which form screwed connections with corresponding counter-threads on the proximal and distal connecting parts. A wedging arrangement is used to lock the parts against relative twisting.
• WO-A-01/70145 discloses apparatus for spinal fixation comprising a multipart core and upper and lower platform members. Coaxial connection between the parts of the core is accomplished by insertion of an end of an inner core part into a bore in an outer core part, and crushing the outer core part using a threaded nut so that it grips the external surface of the inner core part.
• While the prior prosthetic devices have gone a long way toward helping patients with bone or joint disorders, several needs remain unmet. One need is for a viable long bone prosthesis or substitute. Another need is for an adjustable length prosthesis that allows for easy and ready adjustmentsin situ once the prosthesis has been implanted within the patient.
• In order to address these needs, the present invention contemplates an adjustable long bone prosthesis that can be readily adjusted within the patient to account for discrepancies in the measurement of a long bone to be replaced. The inventive prosthesis includes a first elongated component having a first end configured to engage a first prosthetic joint, such as an elbow joint, and an opposite end having a first articulating portion. A second elongated component is provided having a second end configured to engage a second prosthetic joint, such as a shoulder joint, and an opposite end having a second articulating portion. The two articulating portions are adjustably mateable with each other so that the combined length of said first and second components can be adjusted to approximate the length of the long bone to be replaced.
• The present invention contemplates an adjustable prosthesis that permits adjustment of the length of the prosthesisin situ most preferably, the invention has application for the replacement or substitution of a long bone of a patient, such as the humerus bone. However, the adjustability features of the present invention can be implemented for other prosthesis, such as prosthetic joints. In one aspect of the invention, the adjustable prosthesis utilizes a threaded collet and tapered nut for providing a pressure engagement between articulating components of the prosthesis.
• In one feature of the invention, the first articulating portion includes an elongated stem, while the second articulating portion includes a collet having a threaded outer surface and an inner surface arranged for pressure engagement of the stem therein. The prosthesis further includes a lock nut having a tapered inner threaded surface configured for threaded engagement with the threaded outer surface of the collet. With this feature, threading the lock nut onto the collet compresses the collet about the elongated stem therein to fix the relative axial and rotational position of the two components.
• In accordance with the invention, the second articulating portion also includes a blind bore sized contiguous with the inner surface of the collet. The blind bore is sized for a close running fit with the elongated stem so that the stem can be freely rotated or translated relative to the bore. In certain embodiments, the stem and bore can define an interdigitating interface between them. The interdigitating interface can be configured for indexed rotation or translation of the stem relative to the bore. Alternatively, the interdigitating interface can be configured to prevent or limit relative rotation or translation between the components. In a specific embodiment, the interdigitating interface includes a number of axial ribs on the stem and the bore.
• The inner surface of the collet and the outer surface of the stem can collectively include a surface treatment configured to enhance the pressure fixation or engagement of the collet about the stem. This surface feature can be in the form of an interdigitating interface between the components or in the form of deformable projections.
• The invention provides a prosthesis which can be adapted as a long bone replacement. A further object resides in features of the invention that allow for easy adjustment of the length of the prosthesis within the patient.
• One benefit of the invention is realized in the ability to permit length adjustment and achieve a strong engagement in the adjustable length prosthesis. Another benefit is that the components can be readily adapted for many skeletal structures and replacement of a number of long bones. A further benefit is that the components of the prosthesis can be "mixed and matched" among a selection of components depending on the patient's anatomy.
• Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:
• FIG. 1 is a perspective view of an adjustable long bone prosthesis in accordance with one embodiment of the present invention.
• FIG. 2 is a top elevational view of the prosthesis shown inFIG. 1.
• FIG. 3 is an enlarged end perspective view of a collet and locking nut feature of the prosthesis shown inFIGS. 1 and2.
• FIG. 4 is an enlarged perspective view of an end portion of a lower articulating portion of the prosthesis shown inFIGS. 1 and2.
• FIG. 5 is a side cross-sectional view of the articulating components and lock nut prior to engagement with each other.
• Referring now toFIGS. 1 and2, anadjustable prosthesis10 is depicted as including a lower articulatingportion12 that is configured for adjustable articulating engagement with an upper articulatingportion14. In accordance with the illustrated preferred embodiments, theprosthesis10 is intended as a humeral prosthesis. Thus, the two articulatingportions12 and14 are intended to approximate the humerus bone of the patient. For the purposes of the following description, the lower articulating portion may be referred to as the elbowhumeral component12 while the upper portion may be referred to as the shoulderhumeral component14. It is of course understood that the upper and lower portions can be modified depending on the nature of the prosthesis. For instance, the prosthesis can be provided for replacement of a different long bone. Likewise, one or both of theportions12 and14 can be modified for engagement within an existing bone. For instance, theprosthesis10 can be implemented as a substitute for middle portion of a long bone.
• In the illustrated embodiment, the elbowhumeral component12 includes an elbow joint element18 formed at the free end of the component. Similarly, the shoulderhumeral component14 includes a shoulderjoint element20 formed at its free end. In a specific embodiment, the elbow joint element18 can form part of the modular elbow disclosed in US-6290725. Likewise, the shoulderjoint element20 can be configured as shown is US-5314479.
• As shown inFIG. 1, the elbowhumeral component12 includes a lowerelongated member25, while the shoulderhumeral component14 includes an upperelongated member27. The upper member defines ablind bore30 extending into one end thereof (i.e., the end opposite the shoulder joint element20). The lowerelongated member25 defines anelongated stem32 that is sized for a close running fit within thebore30. Thestem32 is defined at the end of themember25 opposite the elbow joint element18.
• In accordance with one important aspect of the invention, the upperelongated member25 further defines acollet portion35 integrated into the end of theelongated member27 at theblind bore30. As shown inFIG. 3, thecollet portion35 is defined by a number ofcollet fingers37 separated byslots38. Thefingers37 define abore39 that is substantially contiguous with the blind bore30 in the upperelongated member27. Preferably, thebore39 and the blind bore30 are each formed at the same diameter and most preferably in a common drilling and reaming operation.
• As shown inFIGS. 1 and2, alock nut16 is disposed over thecollet portion35. More specifically, thecollet portion35 defines a series ofouter threads41 along its exterior surface and extending over a substantial portion of the length of thecollet portion35. Thelock nut16 includes matinginner threads44 that are configured for threaded engagement with theouter threads41 of the collet portion.
• In accordance with the present invention, the combination of thelock nut16,collet portion35 and stem32 of the lowerelongated member25, provides for a pressure engagement or lock between the lower and upper articulatingcomponents12 and14. More specifically, the inner surface58 (FIG. 5) of thecollet portion35 is compressed against the outer surface of thestem32 by threaded engagement of thelock nut16 onto theouter threads41 of thecollet portion41. To achieve this pressure lock function, it is necessary that thecollet portion35 be radially compressible to attempt to reduce the diameter of thebore39 to a dimension less than the outer diameter of thestem32. In the preferred embodiment, this reduction is accomplished by making the lock nut16 a tapered nut. More specifically , theinner bore35 of thenut16 is tapered from a larger diameter at the end facing thecollet portion35, to a smaller diameter at the end facing the elbowhumeral component12. Most preferably, the smaller diameter portion of the tapered bore45 withinlock nut16 is small enough to produce an interference fit between thecollet portion35 and thestem32. In this way, the amount of pressure fixation force can depend upon the degree which thelock nut16 is threaded over thecollet portion35.
• In accordance with the preferred embodiment, it is anticipated that thecollet portion35 includes outer threads that extend along the length of the collet portion a sufficient distance to accept the length of the inner threads of thelock nut16. With this approach, the collet portion can be completely hidden behind thelock nut16 to reduce the likelihood of soft tissue injury once the prostheses has been implanted within the patient.
• In the most preferred embodiment, theinner bore45 of the nut is tapered. In alternative embodiments, the outer threadedsurface41 of thecollet portion35 can be tapered with theinner threads44 of thelock nut16 having a constant diameter. Similarly, theinner bore39 of the collet portion can be tapered with the outer diameter of the collet portion and the inner diameter of the lock nut remaining constant. In any of these configurations, threading thelock nut16 onto thecollet portion35 exerts a radially inward compressive pressure or force against thestem32, thereby locking the two articulatedcomponents12 and14 together. In a specific embodiment, the angle implemented can be a Morse angle of about 1 to 3°. Both thelock nut16 and thecollet portion35 can utilize the same Morse angle to enhance the fixation between the lock nut and the collet portion.
• In accordance with a preferred embodiment of the invention, thestem32 is sized for a close running fit in both rotation and translation relative to the blind bore30 and collet bore39. In other words, in the preferred embodiments the elbowhumeral component12 can articulate relative to the shoulder humeral14 in both angular and extensional directions. In this way, the length of theprosthesis10 can be adjusted to approximate the length of the patient's original bone, and the angular orientation between thejoint elements18 and20 can be adjusted by rotating thestem32 relative to theblind bore30. In a specific embodiment, the close running fit between the stem and bore can be about 0.10 inches (2.5 mm) radially.
• In order to assist in setting the angular orientation between the twocomponents12 and14, interdigitating ofribs51 on thestem32 andribs52 on the blind bore30 can be implemented (FIGS. 3-5). Preferably, with this embodiment, theribs51 and52 only barely project above their corresponding surfaces. In this way, the interdigitatingribs51 and52 do not impede relative rotation between the elbowhumeral component12 and the shoulderhumeral component14. Instead, theribs51, 52 can offer a slight resistance at periodic intervals of angular movement, and can even generate a clicking sound indicative of a particular indexed angle of rotation. For example, the interdigitatingnibs51 can be disposed at 10° intervals around the diameter of thestem32. A single rib can be implemented on one of the two components, particularly where only an audible indication is necessary. Alternatively, a greater number of ribs can be implemented on one element relative to the other. Preferably, theribs51, 52 can have a height slightly greater than half the running radial fit, such as 0.06 inches (1.5 mm) in a specific embodiment
• In certain instances, relative rotation between the twocomponents12 and14 is undesirable, and even detrimental. In embodiments of the prosthesis of this type, the interdigitatingribs51 and52 can be more prominent with respect to thecorresponding elements32, 30. In other words, theribs51 and52 can be configured so that no rotation between the two components is possible. In some embodiments, an interlocking spline configuration can be implemented for theribs51 and52. With this configuration, the blind bore30 can be provided with either elongated channels or upstanding splines, while thestem32 can be complimentary configured with upstanding splines or channels. Thus, once thestem32 is extended into the blind bore30, the angular position between thecomponents12 and14 is fixed, regardless of the position of thelock nut16 on thecollet portion35. In alternative embodiments, the shapes of the bore and stem can be non-circular complementary shapes, such as oval or square, to prevent relative rotation.
• In a similar fashion, circumferential ribs can be provided on the articulating components to provide for indexed length adjustment. Thus, theribs51 on thestem32 can extend around the circumference of the stem at predetermined axial intervals. Therib52 in the blind bore30 can be modified to be a single rib or projection at the mouth of the bore that provides minimal resistance to penetration of thestem32 into thebore30. Again, the modified ribs can provide an audible click to indicate indexed length positions of thehumeral components12, 14.
• As a further option, the stem and bore can be provided with a retention feature that helps hold the components together prior to implantation in the patient. This retention feature can include a circumferential rib at the end of the stem and a corresponding circumferential rib at the end of the bore, where the two ribs provide an interference fit. The interference is overcome when the stem is initially pushed into the bore, and is sufficient to hold the stem in place when subject to normal manipulation.
• Thestem32 can include asurface feature55 configured to enhance the pressure fixation and grip of thecollet portion35 on thestem32. A similar surface feature can be implemented on theinner surface58 of thecollet portion35. A variety of surface treatments are contemplated, provided that the treatments enhance or increase the pressure fixation between the components when thelock nut16 is fully threaded onto thecollet portion35. For instance, thesurface feature55 can include a knurling pattern, circumferential and/or longitudinal splines, or deformable protrusions. A similar treatment can be provided toinner surface58 of thecollet portion35.
• As should be appreciated from the foregoing description, and upon viewingFIGS. 1 and2, as well as the exploded view inFIG. 5, theadjustable prosthesis10 can be easily adjusted when the prosthesis is situated in the patient. In order to effect adjustments, it is only necessary to leave the lockingnut16 un-tightened relative to thecollet portion35. The overall length of theprosthesis10 can be adjusted by changing the amount that thestem32 extends into theblind bore30. Thus, the overall combined length of thehumeral components12 and14 can be specifically calibrated within the patient to ensure a prosthesis length that is anatomically correct for the particular patient. In addition, the angular positioning between the twojoint elements18 and20 can be adjusted.
• Once the components have been situated at their prescribed positions, thelock nut16 can be threaded onto thecollet portions35. Preferably, thelock nut16 is configured to engage a wrench, by including anouter wrench surface46. In one preferred feature of the invention, theouter wrench surface46 includes only roundededges47, to reduce the risk of trauma to the surrounding soft tissue. As shown inFIG. 3, thewrench surface46 can be configured to accommodate a hex wrench. Of course, other wrench surface configurations can be provided to engage a variety of wrenches.
• It is preferable that the coarseness of the engaging ofthreads41 and44 be calibrated so that excessive rotation of thelock nut16 is not required to accomplish a solid fixation. On the other hand, a certain fineness to the threads is also desirable to enhance the fixation between the components and reduce the likelihood that thelock nut16 will become unthreaded from thecollet portion35. As further insurance against this undesirable event, a biocompatible adhesive can be utilized between thelock nut16 andcollet35. In addition, the Morse taper feature can be incorporated between the locking and/or articulating components. As a further alternative, thelock nut16 can be supplemented by an additional lock nut that can be threaded against thelock nut16 to prevent its counter-rotation.
• A further feature of the present invention can be appreciated upon review ofFIGS. 2 and5. In particular, the shoulderhumeral portion14, and specifically its upperelongated member27, exhibits a particular outer diameter. Preferably, the outer diameter approximates the diameter of the long bone being replaced. In addition, this upperelongated member27 accounts for the majority of the overall length of theprosthesis10. In order to maintain the overall continuity of the prosthesis, thelock nut16 can have an outer dimension that is not substantially greater than the diameter of theelongated member27, and is actually preferably smaller in diameter than that portion. Likewise, the lowerelongated member25 of the elbowhumeral component12 can have an outer diameter that is less than or equal to the outer diameter of the upperelongated member27. In this way, the overall profile of theprosthesis10 is smooth and presents as little disruption to the surrounding tissue of the patient than is necessary.
• Most preferably, thecomponents12 and14 andlock nut16 are formed as a biocompatible metallic material, such as titanium. It is also preferable that the material be x-ray opaque so that the continued viability of the prosthesis can be judged at subsequent x-rays. In a specific embodiment in which theprosthesis10 is intended for replacement of the humerus, the shoulderhumeral component14 can have an outer diameter of about 0.75 inches (20 mm) and a length from the base of the shoulderjoint element20 to the base of thecollet portion35 of about 4.25 inches (110 mm). Thecollet portion35 can extend about 1.0 inches (25 mm) from the end of the upperelongated member27, and have an outer diameter of about 0.5 inches (13 mm) and a bore diameter of about 0.4 inches (10 mm).
• The elbowhumeral component12, and particularly thelower member25, can have a length from the elbow joint element18 to the base of thestem32 of about 2.1 inches (53 mm) with a diameter of 0.55 inches (14 mm). Thestem32 has a diameter slightly less than the diameter of thebore39, and a length from the end of the lowerelongated member25 of about 1.5 inches (38 mm). Thelock nut16 can have a length of about 0.95 inches (24 mm).
• It should be appreciated that while the important feature of the present invention allowsin situ adjustments to the prosthesis when it is intentionally implanted, this same feature permits subsequent adjustments of the length and /our orientation of the prosthesis. Thus, if a certain amount of slippage occurs over time or due to a traumatic load, thelock nut16 can be loosened, thereby allowing readjustment of the upper andlower compotes25 and27. It should also be appreciated that the modular aspect of theprosthesis10 allows a lot of theother components12 and14 to be replace. For instance, if a new elbow joint is developed which requires a different elbow joint element, the elbowhumeral component12 can be replaced with the newly designed component. This same capability is beneficial during the initial surgery, since it allows mixing and matching of various shoulder and elbow components as dictated by the patient's anatomy.
• It is possible that both of theelongated members25 and27 can be engaged within the intramedullary canal of a sustained bone of a patient. Thus, one end of the prosthesis can be engaged to a bone segment, while the other end is engaged to a joint element. Similarly, both ends can be engaged to a bone segment, with the prosthesis providing a replacement for a middle portion of an existing bone. As indicated above, the illustrated embodiment concerns a replacement for the humerus, while the principles of the invention can be applied to other bones, especially human long bones such as the femur, tibia, fibula, ulna, and radius.
• The blind bore30 and stem32 are described above as integrated into theupper member27 andlower member25, respectively. It should be apparent that the bore and stem can be switched to the opposite member.

Claims (7)

1. An adjustable long bone prosthesis (10) comprising:

   a first elongate component (25) having a free end and an opposite end having an elongate stem (32); and

   a second elongate component (27) having a free end and an opposite end having an articulating portion (14) which has a bore (30) formed in it which is sized so that the elongate stem is a close running fit therein and thereby adjustably mateable so that the first and second components define an adjustable length to approximate the length of a long bone;

   characterised in that the articulating portion includes a collet (35) having a threaded outer surface (41) and an inner surface (58) arranged for pressure engagement with the elongate stem when the collet is compressed about the stem, the prosthesis including a nut (16) which has a threaded inner surface (45) for threaded engagement with the outer surface of the collet, with at least one of the collet and the nut defining a tapered surface which is arranged so that threading the nut onto the collet compresses the collet about the elongate stem;
   andin that the facing surfaces of the elongate stem and the bore define an interdigitating interface (51, 52) between them which is configured for indexed rotation of the stem relative to the bore.
2. The adjustable long bone prosthesis according to claim 1, in which the threaded inner surface (45) of the nut (16) provides the tapered surface.
3. The adjustable long bone prosthesis according to claim 1, in which at least one of the elongate stem (32) and the inner surface of the collet (35) define a surface feature (55) configured to enhance the pressure engagement between them.
4. The adjustable long bone prosthesis according to claim 3, in which the elongate stem (32) and the inner surface of the collet (35) define an interdigitating interface (55) between them.
5. The adjustable long bone prosthesis according to claim 1, in which the outer dimension of the nut (16) is no greater than about the largest outer diameter of the first component (25) and the second component (27).
6. The adjustable long bone prosthesis according to claim 1, in which the first component (25) includes an elongate body (12) between the free end and the stem (32), the body defining an outer dimension that is greater than an outer dimension of the stem.
7. The adjustable long bone prosthesis according to claim 1, in which the nut (16) has an outer surface (46) configured for engagement with a wrench, and preferably includes only rounded edges (47).

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